docs: def get_frame(self, index):
          ''' Get video frame at the specified index.
  
          Args:
              index (int): Positional index of the desired frame.
          '''
  
          frame_num = self.frame_index[index]
          onset = float(frame_num) / self.fps
  
          if index < self.n_frames - 1:
              next_frame_num = self.frame_index[index + 1]
              end = float(next_frame_num) / self.fps
          else:
              end = float(self.duration)
  
          duration = end - onset if end > onset else 0.0
  
          return VideoFrameStim(self, frame_num,
                                data=self.clip.get_frame(onset),
                                duration=duration)